<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <ul>
        <li>111</li>
        <li>222</li>
        <li>333</li>
    </ul>


    <script>
        /* 
            需求：复制 li 333
         */

         const ulObj = document.querySelector("ul")
         const liObj = ulObj.children[2].cloneNode(true);       //加true克隆内容
         console.log(liObj)
         ulObj.appendChild(liObj)
    </script>

</body>

</html>